as in treacherous
formal
causing harm in a way that is gradual or not easily noticed
Most people with this insidious disease have no idea that they are infected. an insidious enemy
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Recent Examples of insidious
Another of measles’ worst and most insidious outcomes is subacute sclerosing panencephalitis (SSPE), an untreatable neurodegenerative condition that can take nearly a decade to manifest.
—
Katherine J. Wu, The Atlantic, 24 Feb. 2026
Political discussions on social media are often dominated by competing attributions of more and more insidious motives to people on the other side.
—Mark Schroeder, The Conversation, 20 Feb. 2026
Here, the joyous and the boisterous contrast the dark and insidious, allowing viewers to both be delighted by colorful street scenes and celebrations while the rot of fear creeps in, as the details surrounding our main character (Wagner Moura) and his predicament are revealed.
—Megan Spurrell, Condé Nast Traveler, 11 Feb. 2026
The abstracting reached its pinnacle (nadir) with an insidious Alexa ad featuring Chris Hemsworth and wife Elsa Pataky.
—Steven Zeitchik, HollywoodReporter, 9 Feb. 2026
See All Example Sentences for insidious
Katherine J. Wu, The Atlantic, 24 Feb. 2026
Political discussions on social media are often dominated by competing attributions of more and more insidious motives to people on the other side.
—Mark Schroeder, The Conversation, 20 Feb. 2026
Here, the joyous and the boisterous contrast the dark and insidious, allowing viewers to both be delighted by colorful street scenes and celebrations while the rot of fear creeps in, as the details surrounding our main character (Wagner Moura) and his predicament are revealed.
—Megan Spurrell, Condé Nast Traveler, 11 Feb. 2026
The abstracting reached its pinnacle (nadir) with an insidious Alexa ad featuring Chris Hemsworth and wife Elsa Pataky.
—Steven Zeitchik, HollywoodReporter, 9 Feb. 2026
See All Example Sentences for insidious
Recent Examples of Synonyms for insidious
Adjective
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The corridor has major safety issues, such as speeding and treacherous turns out of strip malls.—
Carolyn Gusoff,
CBS News,
6 Mar. 2026
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The sport has only grown since then, with paths to championships proving more treacherous than ever before.—
Jace Frederick,
Twin Cities,
2 Mar. 2026
Adjective
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Among California’s most upscale neighborhoods, a subtle reshuffling has taken place in recent decades, with the highest home values migrating from Northern California to Southern California.—
Terry Castleman,
Los Angeles Times,
12 Mar. 2026
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Key Takeaways The symptoms of transthyretin amyloid cardiomyopathy (ATTR-CM)—a disease of the heart muscle—are often subtle and can mimic those of other heart conditions, especially in the early stages.—
Colleen Doherty,
Verywell Health,
11 Mar. 2026
Adjective
-
Richards and the governor’s office pushed back on false assertions that Newsom and his wife, First Partner Jennifer Siebel Newsom, were stealing money from the state through her office that same day.—
Taryn Luna,
Los Angeles Times,
6 Mar. 2026
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Air power cannot overthrow a government, some say, which is on one level true—a fighter jet cannot take the keys to the presidential palace, after all—and on another level, obviously false.—
Eliot A. Cohen,
The Atlantic,
6 Mar. 2026
Adjective
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In the short term, the company focuses on immediate relief from rising annual maintenance fees, open loans, and the financial fallout from deceptive sales practices.—
Malana VanTyler,
USA Today,
9 Mar. 2026
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Tenet had sued Leapfrog last year over poor grades given to its Palm Beach Network hospitals, calling the grading process unfair and deceptive.—
Cindy Krischer Goodman,
Sun Sentinel,
9 Mar. 2026
Adjective
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For those trying to read Iran’s future, the lesson of 1953 is not just that foreign powers can be perfidious.—
Bobby Ghosh,
Time,
5 Mar. 2026
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Observing human behavior confirms that for some among us, the perfidious lust for unbridled power and the imposition of cruelty in its quest know no bounds and are bereft of human decency.—
A.O. Scott,
New York Times,
3 Feb. 2026
Adjective
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The statement came after Bloomberg reported that Oracle and OpenAI had dropped plans to expand the site, though Oracle said media reports regarding Abilene were incorrect.—
Jordan Novet,
CNBC,
10 Mar. 2026
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He was charged in 2013 with tax fraud for filing an incorrect return tied to a Ponzi scheme operated by another man.—
Nick Penzenstadler,
USA Today,
10 Mar. 2026
Adjective
-
The helpful staff won’t steer you wrong.—
Condé Nast,
Condé Nast Traveler,
7 Mar. 2026
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An officer monitoring a vehicle X-ray machine did not detect anything wrong with the vehicle, but another officer doing a physical inspection spotted the woman’s foot, according to the complaint.—
Alex Riggins,
San Diego Union-Tribune,
7 Mar. 2026
Cite this Entry
“Insidious.” Merriam-Webster.com Thesaurus, Merriam-Webster, https://www.merriam-webster.com/thesaurus/insidious. Accessed 14 Mar. 2026.
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